1. Technical Field
This invention relates to process control systems, and more particularly to noninvasive toroidal-type conductivity sensors.
2. Background Information
Throughout this application, various publications, patents and published patent applications may be referred to by an identifying citation. The disclosures of the publications, patents and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure.
Flow-through electrodeless conductivity sensors are commonly used to measure various parameters of fluids flowing through a manufacturing process. For example, the 871FT™ Conductivity Sensors (Invensys, PLC) are commercially available non-invasive assemblies suitable for a broad range of conductivity and concentration measurement applications. These 871FT sensors are currently available in several line sizes from ½ inch up to 4 inch. They offer a wide choice of wetted parts materials and end connections, including both industrial and sanitary types.
Unlike conventional insertion type sensors, these flow-through sensors are integrated with the process piping, to provide for conductivity measurement in a non-invasive manner. Self-cleaning is provided due to the tubular design geometry. Calibration of the 871FT sensors may be accomplished in-line through use of a built-in calibration port. In-line calibration is beneficial in applications that use aggressive chemicals and others (e.g., pharmaceuticals) in which opening the process line is undesirable due to concerns of potential contamination.
The 871FT sensors may use an optional temperature detector (e.g., Resistive Temperature Detector (RTD)) for automatic temperature compensation. These sensors are compatible with conventional data transmitters and analyzers for use in factory automation networks.
The 871FT sensors rely on metallic end connections to provide electrical continuity with the process solution in order to generate the desired conductivity data. These metallic portions, however, generally render the sensors unsuitable for use in many applications, such as those that use highly corrosive process solutions. For example, corrosive agents commonly used in semiconductor fabrication processes as etchants, such as HF (Hydrofluoric) Acid, are highly reactive with, and thus tend to corrode, metallic components.
These conventional sensors tend also to be incompatible with many high purity processes which may be particularly sensitive to contamination from metals.
Thus, a need exists for an improved flow-through electrodeless sensor suitable for use in highly corrosive environments and/or in environments that demand a relatively high degree of purity.